This is a picture of my back cover and also the casing I have been using. Is there a way to remove the stain ?

Sorry for the weird viewing angle as I the picture shown in my folder is in potrait mode but it uplodad here to a landscape mode and I wanted to upload one more picture of the casing but somehow can't upload it here.

Sorry for the weird viewing angle as I the picture shown in my folder is in potrait mode but it uplodad here to a landscape mode and I wanted to upload one more picture of the casing but somehow can't upload it here.